cleosrv: Choose free port automatically if nothing is specified

i.e. if there is no CLI parameter or config option then default to 8080 and if that is already taken, try to find a free port.

Locate the cleosrv startup entry point and the code handling its CLI parameters and configuration options. Trace how the listening port is selected; done means an explicitly configured port is preserved, while an unspecified port uses 8080 when available and otherwise selects a free port.

Written by the indexing model from the issue text.